Photoplethysmograph (PPG) sensors such as heart rate monitors or pulse oximeters are widely used in medical, wellness and sports areas. They generally use an artificial light source such as an LED that emits light into the skin of a user. The emitted light is scattered within the skin, where it is absorbed partially by blood. Reflected or transmitted light exits the skin and is captured by a photodetector. As a consequence, the signal of the photo detector is an indication of the blood volume. When the blood stream pulsates, the blood volume in the skin changes. Thus, the signal on the photodetector changes directly in response to the pulsation. Hence, the sensor measures directly a pulse of the user in the skin and can thus determine the actual heart rate of the user at a particular moment.
Blood oximeters, especially pulse oximeters, are widely used for measuring oxygenation of blood of a patient. They provide a simple non-invasive method for monitoring the percentage of hemoglobin which is saturated with oxygen. Continuous monitoring of oxygen saturation via pulse oximetry is a standard care procedure used in operating rooms, post anesthesia care units, critical care units and emergency departments.
A pulse oximeter typically comprises two light-emitting diodes or a set of light emitting diodes that emit light of different wavelengths, typically in the red and the infrared part of the spectrum, respectively. The part of the emitted light transmitted through or reflected by tissue of a part of the patient's body, typically a fingertip or an ear lobe, is collected with a photodetector, usually a photodiode.
Absorption of these different wavelengths differs between oxyhemoglobin and its deoxygenated form, so that from the ratio of the collected red and infrared light, the percentage of hemoglobin which is saturated with oxygen can be determined.
In these current systems for optical heart rate measurement and measurement of oxygen saturation, an LED is typically used as the source of light. The signal-to-noise ratio of the collected light signal (after reflection or transmission by the sample) is limited by the signal to noise ratio of the LED source. For this reason, a relatively high quality LED driver is required. Typically, such LED driver circuits require a signal-to-noise ratio of more than 80 dB.
As systems for optical heart rate and oxygen saturation measurement become more common, cost becomes more important. For example, it is proposed to use pulse monitoring systems in watches. Power consumption therefore becomes more important as well.
Having a lower cost and lower power LED driver becomes therefore more important. There is therefore a need to enable a low cost and low power LED and LED driver combination which enables signals of sufficiently high signal to noise ratio to be obtained. The LED driver is responsible for a significant part of the system cost and system power so that reductions in the complexity of the LED driver are particularly desirable. Similarly, a lower cost driver for a laser diode will produce a more noisy laser diode output, so that same issues arise for other types of light source such as laser diodes.